Your situation isn't the most simple one, as higher end devices that can cope with higher WAN speeds tend to come with higher end WLAN; being more expensive in general.
In theory, the lower end mt7621 devices would cope with your performance requirements - but only if you don't have advanced requirements beyond routing/ NAT/ simple firewalling. As soon as your expectations include VPN or other services with higher CPU requirements, its 2*880 MHz mips cores lose steam very quickly.
Low end mvebu (e.g. Linksys WRT1200AC) might also fit the bill, offering the necessary routing performance combined with more CPU performance than mt7621 can offer.
On the other hand, your use case would be a prime example for x86 (the difficulty/ price raiser for x86 usually tends to be equipping it with multiple good WLAN cards), so basically anything with at least two 1 GBit/s ethernet cards would fit the bill. Be it Atom based mini-ITX, several of the chinese mini PCs with 4+ ethernet cards or the PC Engines APU2/3. Either of those will be able to deal with >900 MBit/s throughput, with quite some performance left for additional tasks, starting around ~130-150 EUR/ USD.